This invention relates generally to seismic surveying, and more particularly to three dimensional imaging based upon the use of omni-azimuth seismic energy sources and directional sensing of seismic scatterers.
To investigate a salt dome or like configuration, both a seismic source and seismic receivers or detectors, such as hydrophones, three component geophones, or three component accelerometers, are suspended in a single borehole. Then, seismic signals are sent from the suspended source, such as an airgun, and received by the receivers to define a more accurate map of the salt dome""s flank configuration than possible with surface-located seismic sources and detectors.
Presently, three-dimensional (3-D) seismic surveys are based upon recording a vertical component of seismic motion reflected from sub-surface seismic reflectors. The 3-D surveys rely on the measurements of travel time from source to reflector, to receiver, and the geometry of source-receiver positions on the surface. This technique requires an even distribution of seismic energy sources and receivers over the entire surface of the geological field. The seismic data is acquired separately at each receiver and it is processed by corrected summing to create an image of the sub-surface.
What is needed is an apparatus and a method for conducting 3-D seismic surveys using directional sensing rotation within a geological structure""s complete vector field that is produced by seismic energy emanating from seismic scatterers in the sub-surface of the geological structure, thereby significantly reducing the need for distribution of seismic energy sources and receivers over the entire surface of the geological structure.
The present invention, accordingly, provides an apparatus and a method for conducting 3-D seismic surveys using directional sensing rotation within a geological structure""s complete vector field. Seismic energy emanating from seismic scatterers in the sub-surface of the geological structure produces sufficient energy for the seismic survey. This technique significantly reduces the need for distribution of seismic energy sources and receivers over the entire surface of the geological structure. To this end, an apparatus for providing a three-dimensional seismic image includes an omni-azimuthal source of seismic energy positioned adjacent to a surface of a geological structure. The source emits a signal of sufficient energy and bandwidth to produce seismic energy from a seismic scatterer in the geological structure. A plurality of arrays of sensors are also provided. Each array has directional sensing receivers aligned in the geological structure for receiving and recording measurement of diffracted seismic energy, to produce a complete vector field. The complete vector field is processed to generate the three-dimensional seismic image.
A principal advantage of the present invention is that the sub-surface geological strata is energized (xe2x80x9cilluminatedxe2x80x9d) by the seismic energy source. The energization causes elastic discontinuities (xe2x80x9cseismic scatterersxe2x80x9d) to diffract the seismic energy as if they were the source of such seismic energy. The receivers measure all diffractions. A recording unit records all measured diffractions. A processing system processes the recorded information to locate the seismic scatterers, thus creating a three-dimensional image of the sub-surface, which image can be interpreted for geological significance.